With the rapid development of hybrid (HEY), plug-in hybrid electric vehicles (HEV), and all-battery electric vehicles (EV), there is an urgent need for a rechargeable battery having a high specific energy, high energy density, high rate capability, long cycle life, and safety. One of the most promising energy storage devices is the lithium-sulfur (Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cell consists of elemental sulfur as the positive electrode and lithium as the negative electrode active materials. The lithium-sulfur cell operates with a redox couple, described by the reaction S8+16Li8Li2S that lies near 2.2 V with respect to Li+/Li0. This electrochemical potential is approximately ⅔ of that exhibited by conventional positive electrodes (e.g. LiMnO4) of a lithium-ion battery. However, this shortcoming is offset by the very high theoretical capacities of both Li and S. Thus, compared with conventional intercalation-based Li-ion batteries, Li—S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Assuming complete reaction to Li2S, energy densities values can approach 2,500 Wh/kg and 2,800 Wh/l, respectively, based on the combined Li and S weights or volumes. If based on the total cell weight or volume, the energy densities can reach approximately 1,000 Wh/kg and 1,100 Wh/l, respectively. However, the current Li-sulfur cells reported by industry leaders in sulfur cathode technology have a maximum cell specific energy of 250-400 Wh/kg (based on the total cell weight), which is far below what is possible.
Despite its considerable advantages, the Li—S cell is plagued with several major technical problems that have thus far hindered its widespread commercialization:    (1) Lithium metal cells still have dendrite formation and related internal shorting issues.    (2) Sulfur or sulfur-containing organic compounds are highly insulating, both electrically and ionically. To enable a reversible electrochemical reaction at high current densities or charge/discharge rates, the sulfur must maintain intimate contact with an electrically conductive additive. Various carbon-sulfur composites have been utilized for this purpose, but only with limited success owing to the limited scale of the contact area. Typical reported capacities are between 300 and 550 mAh/g (based on the cathode carbon-sulfur composite weight) at moderate rates.    (3) The cell tends to exhibit significant capacity decay during discharge-charge cycling. This is mainly due to the high solubility of the lithium polysulfide anions formed as reaction intermediates during both discharge and charge processes in the polar organic solvents used in electrolytes. During cycling, the lithium polysulfide anions can migrate through the separator to the Li negative electrode whereupon they are reduced to solid precipitates (Li2S2 and/or Li2S), causing active mass loss. In addition, the solid product that precipitates on the surface of the positive electrode during discharge becomes electrochemically irreversible, which also contributes to active mass loss.    (4) More generally speaking, a significant drawback with cells containing cathodes comprising elemental sulfur, organosulfur and carbon-sulfur materials relates to the dissolution and excessive out-diffusion of soluble sulfides, polysulfides, organo-sulfides, carbon-sulfides and/or carbon-polysulfides (hereinafter referred to as anionic reduction products) from the cathode into the rest of the cell. This phenomenon is commonly referred to as the Shuttle Effect. This process leads to several problems: high self-discharge rates, loss of cathode capacity, corrosion of current collectors and electrical leads leading to loss of electrical contact with active cell components, fouling of the anode surface, and clogging of the pores in the cell membrane separator which leads to loss of ion transport and large increases in internal resistance in the cell.
In response to these challenges, new electrolytes, protective films for the lithium anode, and solid electrolytes have been developed. Some interesting cathode developments have been reported recently to contain lithium polysulfides; but, their performance still fall short of what is required for practical applications.
For instance, Ji, et al reported that cathodes based on nanostructured sulfur/meso-porous carbon materials could overcome these challenges to a large degree, and exhibit stable, high, reversible capacities with good rate properties and cycling efficiency [Xiulei Ji, Kyu Tae Lee, & Linda F. Nazar, “A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries,” Nature Materials 8, 500-506 (2009)]. However, the fabrication of the proposed highly ordered meso-porous carbon structure requires a tedious and expensive template-assisted process. It is also challenging to load a large proportion of sulfur into these meso-scaled pores using a physical vapor deposition or solution precipitation process.
Zhang, et al. (US Pub. No. 2014/0234702; Aug. 21, 2014) makes use of a chemical reaction method of depositing S particles on surfaces of isolated graphene oxide (GO) sheets. But, this method is incapable of creating a large proportion of S particles on GO surfaces (i.e. typically <66% of S in the GO-S nanocomposite composition). The resulting Li—S cells also exhibit poor rate capability; e.g. the specific capacity of 1,100 mAh/g (based on S weight) at 0.02 C rate is reduced to <450 mAh/g at 1.0 C rate. It may be noted that the highest achievable specific capacity of 1,100 mAh/g represents a sulfur utilization efficiency of only 1,100/1,675=65.7% even at such a low charge/discharge rate (0.02 C means completing the charge or discharge process in 1/0.02=50 hours; 1 C=1 hour, 2 C=½ hours, and 3 C=⅓ hours, etc.) Further, such a S-GO nanocomposite cathode-based Li—S cell exhibits very poor cycle life, with the capacity typically dropping to less than 60% of its original capacity in less than 40 charge/discharge cycles. Such a short cycle life makes this Li—S cell not useful for any practical application. Another chemical reaction method of depositing S particles on graphene oxide surfaces is disclosed by Wang, et al. (US Pub. No. 2013/0171339; Jul. 4, 2013). This Li—S cell still suffers from the same problems.
A solution precipitation method was disclosed by Liu, et al. (US Pub. No. 2012/0088154; Apr. 12/2012) to prepare graphene-sulfur nanocomposites (having sulfur particles adsorbed on GO surfaces) for use as the cathode material in a Li—S cell. The method entails mixing GO sheets and S in a solvent (CS2) to form a suspension. The solvent is then evaporated to yield a solid nanocomposite, which is then ground to yield nanocomposite powder having primary sulfur particles with an average diameter less than approximately 50 nm. Unfortunately, this method does not appear to be capable of producing S particles less than 40 nm. The resulting Li—S cell exhibits very poor cycle life (a 50% decay in capacity after only 50 cycles). Even when these nanocomposite particles are encapsulated in a polymer, the Li—S cell retains less than 80% of its original capacity after 100 cycles. The cell also exhibits a poor rate capability (specific capacity of 1,050 mAh/g(S wt.) at 0.1 C rate, dropped to <580 mAh/g at 1.0 C rate). Again, this implies that a large proportion of S did not contribute to the lithium storage, resulting in a low S utilization efficiency.
Furthermore, all of the aforementioned methods involve depositing S particles onto surfaces of isolated graphene sheets. The presence of S particles or coating (one of the most insulating materials) adhered to graphene surfaces would make the resulting electrode structure non-conducting when multiple S-bonded graphene sheets are packed together. These S particles prevent graphene sheets from contacting each other, making it impossible for otherwise conducting graphene sheets to form a 3-D network of electron-conducting paths in the cathode. This unintended and unexpected outcome is another reason why these prior art Li—S cells have performed so poorly.
Despite the various approaches proposed for the fabrication of high energy density Li—S cells, there remains a need for cathode materials, production processes, and cell operation methods that retard the out-diffusion of S or lithium polysulfide from the cathode compartments into other components in these cells, improve the utilization of electro-active cathode materials (S utilization efficiency), and provide rechargeable Li—S cells with high capacities over a large number of cycles.
Hence, there has been strong and continued demand for batteries capable of storing more energy (Wh/l or Wh/kg) and delivering more power (W/kg or W/l) than current rechargeable Li-ion batteries and Li—S batteries. One possible route to meeting this demand is to utilize divalent magnesium ion (Mg2+), rather than the monovalent lithium cation (Li+) because magnesium enables nearly twice as much charge to be transferred, per weight or volume, as Li+ thus enabling higher energy density. Further, magnesium metal and Mg-containing alloys or compounds are more abundant and readily available, potentially enabling significant cost reduction relative to Li metal batteries. Unfortunately, in general, the cathode active materials capable of storing Mg ions exhibit even lower specific capacity (typically <200 mAh/g and more typically <150 mAh/g) as compared to the current cathode active materials for lithium-ion cells. These cathode active materials proposed for use in a Mg-ion cell include: Chevrel phase Mo6S8, MnO2, CuS, Cu2S, Ag2S, CrS2, and VOPO4; layered compounds TiS2, V2O5, MgVO3, MoS2, MgV2O5, and MoO3; Spinel structured compounds CuCr2S4, MgCr2S4, MgMn2O4, and Mg2MnO4; NASICON structured compounds MgFe2(PO4)3 and MgV2(PO4)3; Olivine structured compounds MgMnSiO4 and MgFe2(PO4)2; Tavorite structured compound Mg0.5VPO4F; pyrophosphates TiP2O7 and VP2O7; and FeF3.
In comparison with the aforementioned compounds, sulfur remains a prime candidate cathode active material for a Mg metal secondary battery using Mg metal (>99.9% Mg) or lightly alloyed Mg metal (containing 70-99.9% Mg in the alloy) as the anode active material. In other words, Mg—S would be an ideal high-energy cell. However, the Mg—S cell is not without a problem. In fact, the Mg—S cell has even more problems than the Li—S cell does. There are some similar issues between Mg—S and Li—S batteries, such as: (i) low active material utilization rate, (ii) poor cycle life, and (iii) low Coulombic efficiency. Again, these drawbacks arise mainly from insulating nature of S, dissolution of S and metal polysulfide intermediates in liquid electrolytes (and related Shuttle effect), and large volume change during battery charging/discharging.
However, there are very serious issues that are unique to Mg—S batteries that Li—S batteries do not have: (a) The Li metal at the anode of the Li—S cell can form a passivating layer that is permeable to Li+ ions, but the passivating layer on the Mg metal at the anode of the Mg—S cell is not permeable to the Mg2+ ions, making the dissolution/re-plating of Mg2+ ions (discharge/re-charge of the Mg metal secondary battery) difficult or impossible in most of the known electrolytes; (b) Due to this reason, there are only a limited number of electrolytes that are compatible with Mg metal at the anode; and (c) Unfortunately, these electrolytes are nucleophilic, making them incompatible with the electrophilic sulfur cathode, which is known to require a non-nucleophilic electrolyte.
Hence, a specific object of the present invention is to provide a rechargeable magnesium-sulfur cell based on rational materials and battery designs that overcome or significantly reduce the following issues commonly associated with Mg—S cells: (a) impermeable passivating layer issue of Mg metal anode; (b) electrolyte incompatibility issue; (c) extremely low electric and ionic conductivities of sulfur, requiring large proportion (typically 30-55%) of non-active conductive fillers and having significant proportion of non-accessible or non-reachable sulfur or metal polysulfides); (d) dissolution of S and metal polysulfide in electrolyte and migration of polysulfides from the cathode to the anode, resulting in active material loss and capacity decay (the shuttle effect); and (e) short cycle life.
By addressing most of the aforementioned issues, the present invention provides a technically feasible rechargeable magnesium-sulfur battery that exhibits an exceptionally high specific energy (Wh/kg) or energy density (Wh/l) for a long cycle life. Thus, one particular technical goal of the present invention is to provide a magnesium-sulfur cell with a cell specific energy greater than 400 Wh/Kg, preferably greater than 600 Wh/Kg, and more preferably greater than 800 Wh/Kg (all based on the total cell weight).
Another object of the present invention is to provide a Mg—S cell that exhibits a high cathode specific capacity (higher than 1,200 mAh/g based on the sulfur weight, or higher than 1,000 mAh/g based on the cathode composite weight, including sulfur, conducting additive or substrate, and binder weights combined, but excluding the weight of a cathode current collector, if present). The specific capacity is preferably higher than 1,400 mAh/g based on the sulfur weight alone or higher than 1,200 mAh/g based on the cathode composite weight. This must be accompanied by a high specific energy and a long and stable cycle life.
It may be noted that in most of the open literature reports (scientific papers) and patent documents, scientists or inventors choose to express the cathode specific capacity based on the sulfur or lithium polysulfide weight alone (not the total cathode composite weight). Unfortunately, a large proportion of non-active materials (such as conductive additive and binder that are not capable of storing metal ions) is typically used in their Li—S cells. For practical use purposes, it is more meaningful to use the cathode composite weight-based capacity value. The instant application makes use of total cathode composite material weight (excluding Al foil current collector, if present) as a basis for calculating the cathode specific capacity for our Mg—S cells.